In injection molding, a significant problem is leakage. Leakage is becoming an increasing challenge as new low viscosity resins continue to emerge. Resins that have a melt flow index (“MFI”) of greater than 80 can excessively leak even with a small gap. However, a tight fit between the stem and the bushing can cause the stem to seize. Also, the manufacturing tolerances that are required to prevent leaks are very exacting and costly.
An approach that has been utilized to address this situation is the use of coatings and/or treatments of not only the valve stem but the valve bushing to prevent resin leakage. This provides for a very expensive approach for dealing with this problem.
Another approach to this situation is disclosed in U.S. Pat. No. 6,840,758 (Babin et al.). This patent discloses a spacer that is compressed between an actuator block and a manifold block. The compression causes the spacer to compress and cause the bushing to prevent seepage from traveling up the valve stem and the bushing. However, if too much pressure is applied to the spacer, then valve stem seizure will occur. Also, too little compression may provide leakage.
Still another approach is disclosed in U.S. Pat. No. 6,159,000 (Puri et al.), which discloses a guide sleeve that has a narrow portion that clings to the outside of the valve stem. However, this guide sleeve does not assert any additional pressure to block the flow of resin, especially low viscous resin having a high MFI.
U.S. Pat. No. 6,729,871 (Sattler et al.) discloses the utilization of a cooled bush that increases the viscosity of the melted thermoplastic material in the gap between the stem and the bush. In this manner, leakage is prevented even when the gap is large; however, additional energy and resources are required to provide the cooling, and in this manner there are also issues created involving maintenance. A similar approach is to require cooling to a back plate to increase resin viscosity and prevent resulting leakage. This again is a very costly approach with regard to not only initial expenditures but also energy costs as well as ongoing maintenance.
U.S. Pat. No. 5,518,393 (Gessner) discloses a bushing having a melt channel for mating with a melt channel in a manifold in which the bushing is housed and with an axial channel in a nozzle body. The bushing is sized to fit within a bore in the manifold in an attempt to reduce the possibility of leakage between the bushing and the manifold. However, there is nothing in this structure that will provide additional constriction on the valve stem to reduce resin leakage in the presence of heat, pressure and a high MFI resin.
U.S. Pat. No. 4,344,750 (Gellert) discloses an electrically heated sprue bushing that is seated in a well in the cavity plate with a centrally extending melt runner passage which branches radially outward with separate channels leading to a number of edge gates in the cavity plate. An air gap is provided to insulate the hot sprue bushing from the surrounding cooled cavity plate and a hollow seal is provided at each gate to convey the melt across the air gap. This heating of the metal applies the pressure to prevent resin flow. This is a feature that requires significant energy consumption as well as more maintenance due to increased complexity.
U.S. Pat. No. 5,885,628 (Swenson et al.) discloses an injection molding nozzle for disposition in a mold. The nozzle is for injecting melt into a cavity of the mold, and includes a body having a through bore extending therethrough for receiving the melt. A nozzle member surrounds the body at a position upstream of the nozzle piece and has an inner surface contacting the body and an outer surface contacting the mold that forms a seal against melt flow upstream from the nozzle member. Swenson et al. does not apply any additional pressure to the valve stem to prevent resin flow when resin is flowing in the nozzle.
U.S. Pat. No. 6,555,044 (Jenko) discloses a bushing held in the manifold by a nut that traps a back-up pad. When this nut is tightened, a metal “O” ring seals tightly to prevent plastic leakage along the bore of the bushing. However, “O” rings eventually wear out so that with the presence of vibration, the nut can loosen up to allow resin to flow.
The present invention is directed to overcoming one or more of the problems set forth above.